Descended from a noble but impoverished Neapolitanfamily, he embraced in his youth a military career, but later forsook it for the service of the Church. Endowed with great energy and very talented, he studied law at Bologna, where he took his doctor's degree, and then entered the service of the papal curia. On 27 February, 1402, Boniface IX made him Cardinal-Deacon of St. Eustachius, and in the following year appointed him legate of Romandiola. On 17 March, 1403, he set out for Bologna, where, until 1408, he proved himself an astute financial administrator of the papal territory, as well as a skilful statesman and able commander. At the same time he was utterly worldly-minded, ambitious, crafty, unscrupulous, and immoral, a good soldier but no churchman. He played an important part in the Council of Pisa (1409), and, when the two popes, Gregory XII of Rome and Benedict XIII of Avignon, were deposed, he conducted the election of Pietro Philarghi, who was elevate to the papacy and crowned as Alexander V. The new pope was entirely under the influence of Baldassare Cossa. The latter supported Louis of Anjou in a military expedition against Ladislaus of Naples. Louis seized on several fortresses in the Ecclesiastical States, and in 1400 captured Rome. Alexander V was now proclaimed pope at Rome, but refused to leave Bologna, where he died on 3 May, 1410. In the hope of procuring an understanding with that pope, Prince Malatesta of Rimini, protector of Gregory XII, begged the cardinals of the Pisan obedience to defer a new election. These cardinals assembled at Bologna would not consent, but, supported by Louis of Anjou and the city of Florence, elected Baldassare Cossa, 17 May, 1410. On 24 May Cossa was ordainedpriest, and on the following day was consecrated and crownedpope, taking the name of John XXIII.

Soon after he ascended the throne, John received an ambassador from Sigismund of Hungary, who wished to confer with him about the political and religious affairs of his kingdom. On 18 May King Ruprecht of Germany, the firm supporter of Gregory XII, died. The electors of Mainz and Cologne wrote informing John that they intended to elect Sigismund, King of Hungary, as King of Germany. As Sigismund had, even before he heard of Ruprecht's death, entered into negotiations with the Pisan pope, John exerted himself all the more readily on his behalf, and on 21 July Sigismund, who had become reconciled with his brother Wenzel of Behemia, was chosen King of Germany. Sigismund's election was also recognized by Gregory XII. In April, 1411, John XXIII advanced with Louis of Anjou upon Rome, where they vigorously prosecuted the war against Ladislaus of Naples, and completely routed him at the battle of Roccasecca (19 May, 1411), but made no use of their victory. Soon afterwards, Louis of Anjou returned to France, thus enabling Ladislaus to rally his troops and strengthen his positions. Subsequently, John began negotiations with Ladislaus in spite of the excommunication of 11 August, 1411. Ladislaus thereupon abandoned the cause of Gregory, and acknowledged John as legitimate pope, in recognition of which the latter withdrew his excommunication, enfeoffed Ladislaus with the Kingdom of Naples, consented to his conquest of Sicily, appointed him gonfalonier, or standard-bearer, of the Roman Church, and gave him financial aid (16 October, 1412).

In conformity with a resolution passed at the Council of Pisa, John had summoned a new council to meet at Rome on 29 April, 1412, for the purpose of carrying out ecclesiastical reforms. He also appointed a number of new cardinals, among whom were many able men, such as Francesco Zarabella of Florence, Pierre d'Ailly, Bishop of Cambrai, Guillaume Fillastre, dean of Reims, and Robert Hallam, Bishop of Salisbury. From the beginning of 1412 conferences and meetings of the clergy had been held throughout France in preparation for this council, among the representatives appointed by the king being Cardinal Pierre d'Ailly and Patriarch Cramaud, created cardinal in 1413. But, when the council was opened in April, there were so very few participants that it had to be prorogued several times. When the sessions finally began, the only thing accomplished was the condemnation of the writings of Wycliff, the council being dissolved in March, 1413. John's regrettable weakness in dealing with Ladislaus of Naples soon led to another attack by the latter upon papal territory. In May, 1413, he invaded the Roman province, and John was compelled to fly with his cardinals. He escaped to Florence, where he sought the protection of Sigismund, King of Germany, then labouring in Northern Italy for the convocation of a general council to put an end to the unfortunate schism. John's legates were authorized to come to an understanding with Sigismund on this matter, and Sigismund took advantage of the pope's predicament to insist on the selection of Constance as the meeting-place of the council. On 30 October, 1413, Sigismund invited Popes Gregory XII and Benedict XIII and all Christendom to attend, and prevailed on John XXIII, with whom he had a meeting at Lodi towards the end of November, to issue the Convocation Bull (9 December, 1413) of the general council to be opened at Constance on 1 November, 1414.

By the sudden death of Ladislaus (6 August, 1414) John's position in Italy was improved, and he could now return to Rome. But the cardinals urgently protested that his presence was needed at the Council of Constance, and that he must adhere to his promise of presiding in person, and direct there the treatment of all ecclesiastical matters. On 1 October, 1414, John set out for Constance with a large following and supplied with ample means, but with heavy heart and anxious forebodings. Timidity and suspicion had replaced the warlike spirit he had shown as cardinal. On his way through the Tyrol he formed an alliance with Frederick of Austria, who was on terms of enmity with Sigismund. John and his nine cardinals made their entry into Constance on 29 October, 1414, and on 5 November the council was opened. The prospects of the Pisan pope became daily more hopeless. The emperor had not bound himself by any permanent obligation towards John. He had needed this pope, as possessing ;the largest obedience, to bring about the council, but, from the summer of 1413, he had come to the conclusion that unity could be promoted only by the abdication or the deposal of all three claimants of the papacy. John at first dominated the council, while he endeavoured to increase his adherents by presents, and, by the aid of spies, to learn the temper of the members. However, the hostility of the council towards him became ever more apparent. The chief spokesmen among his cardinals were Pierre d'Ailly and Fillastre; after Sigismund's arrival even these plainly expressed their opinion that the only way to put an end to the schism was by the abdication of all three popes.

In the second session of the council, John was persuaded to read aloud a formal promise of voluntary abdication of the papacy (2 March, 1415), and to repeat this promise in a Bull of 8 March. But on 20 March he fled secretly from Constance to Schaffhausen in the territory of Duke Frederick of Austria, and thence to Freiburg im Breisgau, which belonged to the Duke of Burgundy, also his adherent. John's flight, in consequence of the great difficulties it caused the council, only increased the hostility towards him, and, while he himself tried to negotiate further concerning his abdication, his supporters were obliged to submit to Sigismund. Formally deposed in the twelfth session (29 May, 1415), John made his submission and commended himself to the mercy of the council. John was accused of the gravest offences in several inimical writings as well as in the formal charges of the council. Undeniably secular and ambitious, his moral life was not above reproach, and his unscrupulous methods in no wise accorded with the requirements of his high office. On the other hand, the heinous crimes of which his opponents in the council accused him were certainly gravely exaggerated. After his abdication he was again known as Baldassare Cossa, and was given into the custody of the Palatine Louis, who had always been his enemy. The latter kept him confined in different places (Rudolfzell, Gottlieben, Heidelberg, and Mannheim). At the forty-second session of the council, 28 Dec., 1417, after Martin V had been elected, the release of Cossa was decreed. It was not, however, till the following year that he recovered his liberty. He then set out for Florence, where Martin V was staying, and did homage to him as the Head of the Church. On 23 June, 1419, the new pope made him Cardinal-Bishop of Tusculum. But Cossa was completely crushed, and died a few months later at Florence, where he was buried in the baptistery beside the cathedral. Cosimo de Medici erected a magnificent tomb to his memory.

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